1. Field of the Invention
This invention relates to electronic systems, and more particularly to heat transfer systems and devices used to transfer heat energy produced by a semiconductor device during operation to an ambient.
2. Description of the Related Art
Semiconductor devices (e.g., integrated circuits) dissipate electrical power during operation, transforming electrical energy into heat energy. At the same time, several key operating parameters of a semiconductor device typically vary with temperature, and reliable device operation within specifications occurs only within a defined operating temperature range. For high performance devices, such as microprocessors, specified performance is only achieved when the temperature of the device is below a specified maximum operating temperature. Operation of the device at a temperature above an upper limit of the operating temperature range (i.e., a maximum operating temperature) may result in irreversible damage to the device. In addition, it has been established that the reliability of a semiconductor device decreases with increasing operating temperature. The heat energy produced by a semiconductor device during operation must thus be removed to an ambient environment at a rate which ensures reliable operation.
Several different types of removable heat sinks are available for conveying heat energy generated within an integrated circuit housed within a pin grid array (PGA) package to a surrounding ambient. FIG. 1 is an exploded view of a known electronic apparatus 10 including a heat transfer apparatus 12 for coupling to a PGA device 14 positioned within a zero insertion force (ZIF) socket 16. PGA device 14 includes a PGA device package housing an integrated circuit (IC), and includes multiple pin terminals arranged across an underside surface providing input/output capability for the IC. Heat transfer apparatus 12 includes a heat sink 18 and a spring clip 20. An upper surface of heat sink 18 includes multiple pins projecting upwardly and arranged in rows. ZIF socket 16 includes multiple holes in an upper surface for receiving the pins of PGA device 14, a handle 22 along one side for operating an internal pin coupling mechanism, and multiple pin terminals arranged across an underside surface for coupling the pin terminals of PGA device 14 to electrically conductive traces of a printed circuit board.
As is common, ZIF socket 16 also includes two latching projections 24A and 24B extending outwardly from opposite side surfaces. Spring clip 20 has two apertures 26A and 26B dimensioned to allow respective latching projections 24A and 24B to pass therethrough. In an assembly operation, PGA device 14 is mounted upon the upper surface of ZIF socket 16. With handle 22 in a raised position, the pin terminals of PGA device 14 are inserted into corresponding holes in the upper surface of ZIF socket 16. Handle 22 is then lowered to actuate the internal pin coupling mechanism of ZIF socket 16.
With PGA device 14 positioned within ZIF socket 16, the underside surface of heat sink 18 is brought into thermal contact with the upper surface of PGA device 14. Spring clip 20 is then installed to hold heat sink 18 in place relative to PGA device 14 and to urge the underside surface of heat sink 18 toward the upper surface of PGA device 14. Spring clip 20 is installed by passing resilient bowed arms 28 of spring clip 20 between adjacent rows of pins on the upper surface of heat sink 18 such that apertures 26A and 26B are located directly above respective latching projections 24A and 24B. Sufficient downward pressure is then applied to portions of spring clip 20 above apertures 26A and 26B such that bowed arms 28 are deformed and latching projections 24A and 24B pass through respective apertures 26A and 26B. Following installation of spring clip 20, deformed bowed arms 28 exert a force between heat sink 18 and ZIF socket 16 which urges the underside surface of heat sink 18 toward the upper surface of PGA device 14.
It is now common to mount integrated circuits to substrates using the well known controlled collapse chip connection (C4) or xe2x80x9cflip chipxe2x80x9d techniques. Device packages including integrated circuits mounted to substrates using the flip chip method are commonly known as flip chip packages.
FIG. 2 will now be used to describe a problem which arises when PGA device 14 of FIG. 1 is a flip chip PGA device. FIG. 2 is a cross-sectional view of a known flip chip embodiment of PGA device 14 of FIG. 1. In the embodiment of FIG. 2, PGA device 14 includes an IC 32 mounted upon an upper surface of a substrate 34 using a flip chip technique, and a cover or lid 36 secured over IC 32. A layer 38 of a thermal interface material thermally couples an upward facing backside surface of IC 32 to an underside surface of lid 36. Lid 36 is attached (e.g., adhesively) to the upper surface of substrate 34 about outer edges of the upper surface of substrate 34, and at locations 40A and 40B in FIG. 2. Multiple solder bumps connect a set of I/O pads on a frontside surface of IC 32 to corresponding bonding pads on the upper surface of substrate 34. Substrate 34 includes multiple electrical conductors connecting pins 42 to bonding pads on the upper surface of substrate 12.
The area of the upper surface of lid 36 may be, for example, about 4 square inches. In contrast, the area of the backside surface of IC 32, thermally coupled to lid 36, may be about 0.3 square inches. Thus when the underside surface of heat sink 18 (FIG. 1) is thermally coupled to the upper surface of lid 36, the effectiveness of the transfer of heat energy from IC 32 to heat sink 18 is heavily dependent upon the thermal resistance, and the heat spreading ability, of lid 36. Further, a substantial amount of the heat energy generated within IC 32 is conducted into substrate 34. For heat energy within substrate 34 to reach heat sink 18 (FIG. 1), the heat energy must travel through the attachment points between substrate 34 and lid 36 about the outer edges of the upper surface of substrate 34, and at locations 40A and 40B in FIG. 2. Heat transfer paths between a portion of substrate 34 adjacent to IC 32 and the heat sink are thus relatively long, and include substantial distances within substrate 34. As a result, the effectiveness of the transfer of heat energy from substrate 34 to heat sink 18 is heavily dependent upon the thermal resistance of substrate 34, as well as the rather uncertain thermal resistances at the attachment points between substrate 34 and lid 36.
It would thus be desirable to have a heat removal apparatus for a flip chip PGA device including a heat sink in more effective thermal communication with both the IC and the substrate of the flip chip PGA device. The desired heat removal apparatus would more effectively remove heat energy both from the IC and the substrate, thereby increasing the reliability of the PGA device.
A heat transfer apparatus is described for coupling to a pin grid array (PGA) device including an integrated circuit and mounted within a socket (e.g., a zero insertion force or ZIF socket). The socket is mounted upon a surface of a printed circuit board (PCB) and includes two latching projections extending from opposite side surfaces. The heat transfer apparatus includes a thermally conductive heat sink and a spring clip for holding the heat sink in position relative to the PGA device. The heat sink may be made from a metal (e.g., aluminum), and may have multiple structures (e.g., fins or pins) extending from an upper surface.
The heat sink has an opening in an underside surface for housing the PGA device and the socket. The heat sink also has a lip surrounding the opening for thermally coupling to the PCB about the socket. The heat sink also has a pair of holes extending through the heat sink from the upper surface of the heat sink into the opening. The spring clip has two side members each adapted for attaching to a different one of the two latching projections of the socket. Each of the pair of holes in the heat sink is positioned to receive a different one of the side members of the spring clip. For example, the holes in the heat sink may be separated by a distance equal to a distance between the latching projections of the socket.
In one embodiment, the PGA device includes a substrate having a substantially flat upper surface opposite the socket. The integrated circuit of the PGA device is mounted upon the upper surface of the substrate such that the integrated circuit is elevationally raised above the upper surface of the substrate. In this case, the opening in the underside of the heat sink includes a first cavity dimensioned to receive the socket and the substrate of the PGA device. The first cavity has an upper wall, and the opening also includes a second cavity in the upper wall of the first cavity. The second cavity is dimensioned to receive the integrated circuit.
The heat transfer may include a first thermal interface layer positioned between the integrated circuit and an upper wall of the second cavity. The heat transfer apparatus may also include a second thermal interface layer positioned between a region of the upper surface of the substrate surrounding the integrated circuit and an adjoining portion of the upper wall of the first cavity. The heat transfer apparatus may also include a third thermal interface layer positioned about the socket between the lip of the heat sink and the surface of the PCB.
The spring clip may include a joining member connected between the two side members. The joining member may be made from a resilient material (e.g., spring steel). In one embodiment, the joining member has two ends and a bowed center section between the two ends. The two ends of the joining member are substantially parallel to one another. Each side member of the spring clip has an upper end and a lower end, and a hole in the lower end dimensioned to allow a corresponding one of the latching projections of the socket to pass therethrough. One of the two ends of the joining member is connected to the upper end of one of the side members, and the other end of the joining member is connected to the upper end of the other side member. The center section of the joining member is bowed toward the lower ends of the side members such that the spring clip is substantially xe2x80x9cMxe2x80x9d-shaped.
The latching projections of the socket may be substantially rectangular. In this case, the holes in the lower ends of the side members of the spring clip may also be rectangular.
The spring clip may be formed from a single piece of resilient material (e.g., spring steel). In this case, the spring clip may have sharp bends at interfaces between the side members and the joining member.
A method for coupling the heat sink to the PGA device includes positioning the heat sink over the PGA device and the socket such that the PGA device and the socket reside in the opening in the underside surface of the heat sink. The lower ends of the two side members of the spring clip are inserted into the holes in the heat sink. The side members of the spring clip are pushed downward along the opposite side surfaces of the socket until the latching projections of the socket pass through the corresponding holes in the lower ends of the side members.
A method for removing the spring clip holding the heat sink in position includes exerting a force between upper ends of two side members extending from the holes in the heat sink. The force causes the upper ends of the two side members to move toward one another, and the lower ends of the side members to move away from one another. The moving of the lower ends away from one another allows the latching projections of the socket to exit the holes in the lower ends of the side members, thus detaching the spring clip from the socket.